1. Field of the Invention
The present invention is in the field of television transmitters and, more particularly, is directed to a small, lightweight, inexpensive, portable lowpower television transmitter having a relatively small amount of circuitry yet providing high quality television transmission over medium distance ranges.
2. Prior Art
Over the past several years there have been significant advances made in the miniaturization of tv cameras and tv receivers. For example, there are many models of tv cameras presently on the market which utilize CCD or MOS arrays and which can be held comfortably by an operator. Furthermore, costs have been reduced to the point where such cameras have become readily affordable for home entertainment use, particularly in connection with video recorders. Also, a tv camera which can be held comfortably in the palm of a single hand is commercially available from Polnix, Inc., of California. Miniaturization of tv receivers has been even more dramatic. As the most prominent example, tv receivers having wristwatch type screens plus a receiver box which can be worn on the viewer's belt are presently commercially available from at least one Japanese manufacturer.
Television transmitters, on the other hand, have not kept pace with the latter advances. For purposes of consistency, a television transmitter, as described herein comprises the equipment which receives the video signal plus sync pulses (hereinafter referred to simply as the video signal) and provides an R.F. carrier modulated by the video to a suitable transmitting antenna. The transmitter may be defined to additionally incorporate the antenna itself. Also, the television transmitter includes the circuitry which receives the audio, modulates it onto a carrier or subcarrier, and transmits it along with the video modulated R.F.
As an example of a standard broadcast television transmitter, a 100 watt system, which is not considered to be overly powerful, would be approximately the size of a huge cabinet (e.g., 2.2 meters in width X 2.3 meters in height.times.0.7 meters in depth), weighing approximately 1 ton, cost approximately $70,000 and have a range of 25 km. Such a system, obviously is not portable.
Portable television transmitters are commercially available, but they are too complex, too expensive, and too heavy. Thompson C.S.F., a French company, markets a portable transmitter weighing 10 kilograms and providing 1 watt of power. The latter portable transmitter operates on a frequency of 1.2 gigahertzs with FM modulation. Inasmuch as all commerical television receivers throughout the world operate on amplitude modulated video signals in the megahertz range, the Thompson portable transmitter is not suitable for transmitting directly to commercial television receivers but can transmit only to specially adapted receiving equipment. The Thompson portable transmitter has a size of approximately 30 centimeters width.times.25 centimeters height.times.35 centimeters depth, costs in the neighborhood of $30,000 and has a range of 5 kilometers.
Rhode & Schwarz, GMbH of Germany also markets a portable tv transmitter. That transmitter weighs about 25 kilograms, provides an amplitude modulated signal with a carrier between 470 and 860 megahertz at 5 watts of power and costs approximately $30,000.
As is apparent, the size and costs of present day tv transmitters makes them unsuitable for a wide variety of applications and for use by consumers in connection with home entertainment.
A composite video signal includes the synchronization pulses and the video information, which includes luminance and chrominance signals (for color) or only luminance (for black and white). A television transmitter typically includes a dc restoration circuit, also known as a black level clamping circuit, for properly positioning the video signal relative to 0 volts dc. For the PAL (Germany) and NTSC (U.S. and Japan) systems, the video information is negative and the horizontal sync pulses are positive. In the SECAM system (France) the video information is positive and the horizontal sync pulses are negative.
A transmitter receives the composite signal via cables, rebroadcast receivers, or microwave relay links. The distortion that can occur, especially to the synchronizing pulses, must be taken into account. Standard transmitters convert the incoming signal into a standard television waveform, regenerate the sync pulses and stabilize the video level. The composite video signal often varies in its dc level and therefore the picture.
One type of prior art dc restoration circuit typically operates by reinserting the d.c. component on the tips of the synchronization signal, using relative values required by signal standards. This type of reinsertion is subject to noise impulse interference and is not accurate. Consequently, it has been necessary to add several compensation circuits to avoid such malfunctions. These additional circuits further increase the size and complexity of the transmitter and are themselves liable to additional errors. As another example, some receivers include separate synchronous signal regenerators for correcting the sync pulses as well as establishing the proper dc level. See for example Electronics Engineers' Handbook, 2nd Ed., 1982, McGraw-Hill, Inc., FIG. 21-16, pp. 21-32. Clamping circuits should be used for dc restoration in that they are more accurate, faster and less liable to noise interference. However, they are infrequently used because of their complexity and cost.
In a typical television transmitter, the composite video modulates an R.F. carrier using amplitude modulation, and the audio signal FM modulates an R.F. carrier which differs from the video carrier by, 4.5 to 6.5 megahertz. A diplexer or combiner is used to combine the two modulated signals.
The standard commercial broadcast tv transmitter separately modulates the video and audio on R.F. carriers separated by, e.g., 5.5 MHz and then combines the two modulated signals in a diplexer or combiner. Another technique described in the prior art is to modulate the audio onto a 5.5 MHz carrier and combine it in a diplexer or combiner with the video modulated R.F. carrier, with the output being amplified and resulting in the video and audio modulated R.F. carriers (e.g., 503 MHz and 508.5 MHz) being 5.5 MHz apart.
In either of the above cases, a diplexer or combiner is required and such devices are complicated, expensive and involve a considerable amount of circuitry and therefore are not conducive to miniturization. Examples of such devices are shown in FIGS. 21-17 and 21-18 of Electronics Engineers' Handbook, 1982.
Many adjustments to a television transmitter are necessary in order to be certain that a proper signal is being transmitted. For example, there are several tuned circuits which must be adjusted to insure that they are tuned to the carrier frequency for the particular channel being transmitted; the dc restoration circuitry, video and audio gain circuits, and sync stretching circuits must also be adjusted. Typically, a television transmitter includes several complex, bulky and expensive testing circuits to permit proper adjustment of the transmitter. Examples of such test circuits include a vector scope, a wide band oscilloscope, a color bar reference test generator, calibrated R.F. power meters, and S.W.R. meter, and a spectrum analyzer.